Light obviously needs to enter solar cell structures, which often means transparent contacts are needed to allow light entry while still providing properly positioned electrodes. The materials utilized must have the high conductivities （e.g., > 5 X 102 siemens/cm）， high transparencies, band edge positions, and workfunctions needed to serve as ohmic contacts. Since absorption in these materials is by the free carrier intra-band process 1 of Figure 2.11, there is a trade-off between conductivity and light transmission arising from doping （actually, alloying at the concentrations used）. There is also a trade-off between resistance and transmission with film thickness. Transparent conductive oxide （TCo） materials can meet these trade-off requirements. TCos that have been examined for transparent contact purposes include indium oxide, tin oxide, indium tin oxide （ITo）， zinc oxide, aluminum zinc oxide （AZo）， gallium indium oxide, indium zinc oxide, gallium indium tin oxide, and zinc indium tin oxide. Among these, tin oxide, ITo, and zinc oxide are the most prevalent in actual use.39 Energy band edge position and workfunction information for ITO materials are given in Figure 3.26, along with the corresponding information for several AZo materials.